Optical fibre communication provides a range of advantages compared with conventional “copper-wire” communication, including higher transmission speed and wider bandwidth. The transfer of information is often achieved by superimposing (modulating) the information onto an optical carrier using an electro-optic modulator. Mach-Zehnder type interferometric intensity modulators (MZM) are used for broadband communication links and MZM devices with very large bandwidths in excess of 40 GHz have been demonstrated.
A standard broadband electro-optic modulator comprises a travelling wave electrode which is located adjacent to an electro-optic waveguide. The electric field carried by the travelling wave electrode alters locally the optical properties of the electro-optic waveguide which in turn influences the propagation of the optical carrier and thus transfers the electrical signal information onto the optical carrier. Such broadband modulators tend to be fairly inefficient, with only modest electrical-to-optical conversion efficiency, which limits their application.
Wireless communications typically do not require the extended bandwidth of which MZM devices are capable. For example, the Personal Communication Systems (PCS) standards requires only 60 MHz bandwidth around a centre frequency of 1.9 GHz, allowing great potential for the optimisation of modulation efficiency through resonant enhancement. Future broadband wireless systems designed to operate at millimeter-wave frequencies may require only 1–2 GHz bandwidth centred around operating frequencies in the range of 5–66 GHz.
The electro-optic modulation of an optical carrier by an electrical signal does not involve a transfer of power; instead the optical modulation achieved is proportional to the field strength across the active region of the device, whether this field is a power carrying propagating wave, or a standing wave. It is thus possible to utilise electrodes to generate resonant standing wave structures to greatly increase the field strength at the cavity and hence enhance the modulation efficiency at that resonance. Several demonstrations of resonantly enhanced MZM devices have been reported with significant improvements in efficiency being achieved. For example, a resonantly enhanced modulator which incorporated two electrical terminals that form single a Fabry-Perot type cavity over the active region, had a reported enhancement in link performance of 10 dB at resonance frequency.
Although the development of a single cavity electrode structure at higher radio frequencies should offer many benefits, several shortcomings have also been identified. Across the resonance, the phase response will vary rapidly with frequency. For electrical signals close to the resonant frequency, this variation should be fairly linear, however, it is expected that electrical signals on the edges of the bandwidth may be significantly distorted. In addition, the effectiveness of a single cavity is limited. The length of a standing wave cavity usually corresponds to a half wavelength of the applied electrical signal which is short compared with the typical length of an arm of a MZM device. Longer cavities, with a length corresponding to an integer multiple of the radio frequency wavelength, are not necessarily associated with a higher effectiveness as the strength of the electric field, which results in the modulation of the optical carrier decaying exponentially along the electrode length.
Frequently it is also required to modulate two or more optical carriers, which are guided in separate waveguides, by the same electrical signal. For particular applications, it may be required to achieve a particular phase offset between the modulated signals. One of such examples includes a device which will eliminate one side band and therefore allow for optical single sideband with carrier (OSSB+C) transmission. OSSB+C transmission has the capacity to mitigate the transmission impairment induced by the chromatic dispersion of the transmission optical fibre. Generation of OSSB+C formatted signals can be made possible by applying two identical modulating electrical signals which have a relative phase offset of 90° onto a modulator with two different electrodes corresponding to two different arms of the MZM interferometer. Inside the optical modulator, an optical signal is split into two optical carriers which are then modulated by those applied electrical signals with a 90° phase shift. If, also, the optical carriers travelling in both arms of the modulator have a relative phase offset of 90° between them, the subsequent optical addition of both modulated carriers results in the cancellation of one sideband. A device for this purpose may typically comprises a Mach-Zehnder type modulator with two optical arms, two electrical inputs and a separate 90° radio frequency hybrid coupler. Previous publications have shown that it is critical to maintain a phase difference of the electrical signals applied through the hybrid coupler to the arms of the modulator to within a maximum error of ±5· in order to maximise the suppression of one sideband. In this configuration, however, achieving 90°±5· phase shift over a very wide frequency band of the electrical signals, which are typically in the GHz range, can be very difficult. A typical electrical signal may be a radio frequency signal having a wavelength of the order of 4 mm and the required mechanical accuracy would correspond to approximately 50 μm, which is difficult to achieve with external connectors.
It is therefore desirable to provide a device which has increased effectiveness (i.e. modulation depth) and/or which allows for the synchronised modulation of the optical carrier in branched waveguides.